[FFmpeg-devel] [PATCH v2] configure: Explicitly check for static_assert

[Martin Storsjö]

On Thu, 21 Mar 2024, Andreas Rheinhardt wrote:

> Andreas Rheinhardt:
>> C11 provides static assertions via _Static_assert and
>> provides static_assert as a convenience define for this
>> in assert.h. MSVC 19.27 declares support for C11, but does
>> not support _Static_assert, but somehow supports
>> static_assert. That's therefore what we use.
>>
>> But apparently there are some old GCC toolchains where
>> _Static_assert is supported, but assert.h does not provide
>> the fallback define. Some fate boxes are affected by this
>> [1].
>>
>> This commit therefore checks whether static_assert works
>> with assert.h included; if not, it errors out. Users like
>> the above can still add -Dstatic_assert=_Static_assert
>> to cflags as a workaround.
>>
>> [1]: https://fate.ffmpeg.org/report.cgi?time=20240321123620&slot=sh4-debian-qemu-gcc-4.7
>>
>> Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt at outlook.com>
>> ---
>> This is what a test without fallback looks like.
>> Posted to gather opinions on what people prefer.
>>
>>  configure | 13 +++++++++++++
>>  1 file changed, 13 insertions(+)
>>
>> diff --git a/configure b/configure
>> index 6d7b33b0ff..c2d2c70c20 100755
>> --- a/configure
>> +++ b/configure
>> @@ -5589,6 +5589,19 @@ check_cxxflags_cc -std=$stdcxx ctype.h "__cplusplus >= 201103L" ||
>>  check_cflags_cc -std=$stdc ctype.h "__STDC_VERSION__ >= 201112L" ||
>>      { check_cflags_cc -std=c11 ctype.h "__STDC_VERSION__ >= 201112L" && stdc="c11" || die "Compiler lacks C11 support"; }
>>
>> +test_cc <<EOF || die "Compiler lacks support for C11 static_assert"
>> +#include <assert.h>
>> +#include <stddef.h>
>> +struct Foo {
>> +    int a;
>> +    void *ptr;
>> +} obj;
>> +static_assert(offsetof(struct Foo, a) == 0,
>> +              "First element of struct does not have offset 0");
>> +static_assert(offsetof(struct Foo, ptr) >= offsetof(struct Foo, a) + sizeof(obj.a),
>> +              "elements not properly ordered in struct");
>> +EOF
>> +
>>  check_cppflags -D_FILE_OFFSET_BITS=64
>>  check_cppflags -D_LARGEFILE_SOURCE
>>
>
> Jan has tested old toolchains and found out that his GCC 4.7 has proper
> C11 headers; so this seems to be unique to Michael's setup. This makes
> me prefer this patch instead of the version with the fallback. (Michael
> can simply add -Dstatic_assert=_Static_assert to his cflags.)
> Of course others are still invited to share their opinions.

Both patches seem to work fine with MSVC 19.27 - I vaguely prefer the v2 
version, which is simpler.


But to me, we could also just revert the change to 
libavcodec/ccaption_dec.c, and declare that we require MSVC 19.28 instead. 
MSVC 19.27, when executed with -std:c11 without -nologo, it prints this:

     /std:c11 is a preview implementation of the ISO C11 standard, and
     we're eager to hear about bugs and suggestions for improvements.
     However, note that these features are provided as-is without support.

And I don't have any specific reasons for wanting to use this compiler - I 
just tested the lowest version that was supposed to be supported earlier 
and noted that it had broken recently. So to me, reverting to requiring 
_Static_assert would be quite ok as well.

// Martin